Akiko Takahashi played drums on 7 albums, beginning with Transi (1995), and concluding with "Against The Meteors" on the new album.
Are there any special circumstances surrounding her departure and [keyboardist/vocalist] Mika's?

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As you already know, Akiko left in March, but her reasons for doing so remain private.
She's still drumming in another band, Danball Bat.
Mika left the band last November because she wants to devote herself to Triton [the keyboard trio-styled band she formed prior to joining Ars Nova].

Her compositions embrace an angular symphonic approach with complimentary abstractions and some neo-classical underpinnings all set to a hard-driving rock pace, tethered to earth by the powerful bass of Kyoko Kanazawa and new drummer Akiko Takahashi.

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Akiko also adds violin on a couple of the tracks.

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Bassist Kyoko Kanazawa and drummer/violinist Akiko Takahashi are both solid players, and complement and support Kumagai's impressive keyboard skills well.

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Backing bandswomen Kyoko Kanazawa (Rickenbacker bass) and Akiko Takahashi (drums) work under the keyboards in a very ELP-ish manner, but with plenty of modern fire and flair that ELP has left far behind.

Bassist Kyoko Kanazawa and drummer/violinist Akiko Takahashi are both solid players, and complement and support Kumagai's impressive keyboard skills well.

...

Backing bandswomen Kyoko Kanazawa (Rickenbacker bass) and Akiko Takahashi (drums) work under the keyboards in a very ELP-ish manner, but with plenty of modern fire and flair that ELP has left far behind.

...

Her compositions embrace an angular symphonic approach with complimentary abstractions and some neo-classical underpinnings all set to a hard-driving rock pace, tethered to earth by the powerful bass of Kyoko Kanazawa and new drummer Akiko Takahashi.

Her compositions embrace an angular symphonic approach with complimentary abstractions and some neo-classical underpinnings all set to a hard-driving rock pace, tethered to earth by the powerful bass of Kyoko Kanazawa and new drummer Akiko Takahashi.

...

Akiko also adds violin on a couple of the tracks.

...

Backing bandswomen Kyoko Kanazawa (Rickenbacker bass) and Akiko Takahashi (drums) work under the keyboards in a very ELP-ish manner, but with plenty of modern fire and flair that ELP has left far behind.

...

Bassist Kyoko Kanazawa and drummer/violinist Akiko Takahashi are both solid players, and complement and support Kumagai's impressive keyboard skills well.